The application reflects a long-term commitment to study the molecular bases for the meiotic abnormalities that result in Down syndrome (trisomy 21), and for the clinical phenotype that results from trisomy 21. The specific aims are to: 1) Examine features of the organization of DNA sequences about the centromere of chromosome 21, and their relation to meiotic abnormalities involving chromosome 21. 2) Isolate genes on chromosome 21 that encode surface proteins responsible for the increased adhesiveness of trisomy 21 fetal fibroblasts; increased dosage for these genes may be responsible for the congenital malformations seen characteristically in Down syndrome. A recombination-based assay will be employed to continue chromosome walking experiments in the pericentromeric region of human acrocentric chromosomes. Single copy DNA probes for the centromeres of individual human acrocentric chromosomes, including chromosome 21, will be isolated to study the parental origin of nondisjunction events, and to study the assortment of individual acrocentric chromosomes in interphase and meiosis. The techniques used in the recombination based assay suggest unique opportunities to study gene mapping and the effects of gene dosage on morphogenesis in procaryotic systems. The procaryotic studies outlined include: 1) gene dosage effects of antiterminator proteins encoded by phasmid P4 on P4 capsid morphogenesis; 2) gene dosage effects of the E. coli recA and supF genes, to improve cloning methodologies; 3) cloning, mapping and chromosome walking among bacterial virulence genes. Cell surface markers encoded by chromosome 21 may mediate the observed increase in adhesiveness among trisomy-21 fetal-lung and endocardial-cushion fibroblasts and, thereby, interfere with normal pulmonary and cardiac development in Down syndrome. Monoclonal antibodies that react with chromosome 21-encoded cell surface markers and that inhibit the rapid aggregation of trisomy 21 cells will be used to isolate genes on chromosome 21 that encode such markers. Isolation of a gene that is responsible for the increased aggregation of trisomy 21 fetal fibroblasts could ultimately be used to study the genesis of congenital heart defects in trisomy 16 in mouse, and in trisomy 21 in man.